F. Rohart

Speed dependence of rotational relaxation induced by foreign gas collisions: Studies on CH3F by millimeter wave coherent transients

The dependence of rotational relaxation rates on the speed of absorbing molecules has been studied by millimeter wave coherent transients for the J,K=1,1–2,1 rotational transition of methyl fluoride (CH3F). A new phenomenological model used to describe such a speed dependence has been introduced. It leads to a quite simple analytical expression for time‐domain transient signals, the Fourier transform of which corresponds to the frequency‐domain line shape (speed‐dependent Voigt profile). The investigations were carried out on mixtures of CH3F with He, Ar, Xe, H2, D2, N2, and O2, yielding parameters which characterize the speed dependence of the observed decay rates and its pressure and temperature dependence. Special emphasis was given to the key role of the mass ratio of collision partners which clearly allowed the relation of the observed nonexponential decay behavior to collisional effects. However, the observations cannot be explained exclusively with consideration of speed‐dependent rates, but must a...

Line frequency shifting in the ν5 band of C2H2

Abstract The pressure induced line frequency shifts of C 2 H 2 in the ν 5 band at 13.7 μm have been measured with N 2 , H 2 , Ar and He as buffer gases. The experiment used a diode laser which was frequency stabilized on the absorption peak of the studied line, and line shifts were determined by monitoring the laser frequency drifts vs the gas pressure. Comparisons have been made with calculated results based on a semi-classical model including adjustable potential parameters.

Velocity effects on the shape of pure H2O isolated lines: complementary tests of the partially correlated speed-dependent Keilson-Storer model.

Complementary tests of the partially correlated speed-dependent Keilson-Storer (pCSDKS) model for the shape of isolated transition of pure water vapor [N. H. Ngo et al., J. Chem. Phys. 136, 154310 (2012)] are made using new measurements. The latter have been recorded using a high sensitivity cavity ring down spectrometer, for seven self-broadened H(2)O lines in the 1.6 μm region at room temperature and for pressures from 0.5 to 15 Torr. Furthermore, the H(2) (18)O spectra of [M. D. De Vizia et al., Phys. Rev. A 83, 052506 (2011)] in the 1.38 μm region, measured at 273.15 K and for pressures from 0.3 to 3.75 Torr have also been used for comparison with the model. Recall that the pCSDKS model takes into account the collision-induced velocity changes, the speed dependences of the broadening and shifting coefficients as well as the partial correlation between velocity and rotational-state changes. All parameters of the model have been fixed at values previously determined, except for a scaling factor applied to the input speed-dependent line broadening. Comparisons between predictions and experiments have been made by looking at the results obtained when fitting the calculated and measured spectra by Voigt profiles. The good agreement obtained for all considered lines, at different temperature and pressure conditions, confirms the consistency and the robustness of the model. Limiting cases of the model have been then derived, showing the influence of different contributions to the line shape.

Microwave spectrum of nitromethane internal rotation Hamiltonian in the low barrier case

Abstract The microwave spectrum of nitromethane has been measured in the 8–80 GHz range. More than 130 lines of ground and excited states are assigned. Symmetry considerations of the low barrier case lead to a Hamiltonian including centrifugal distortion effects. It is shown that the usually used coupling term between internal and overall rotations has to be modified in order to get a good fit of the excited states spectrum. This result seems to be similar to the inertial defect observed in planar molecules. The influence of the V 12 term of the m = 6 state has been observed.

Foreign gas relaxation of the J=0→1 transition of HC15N. A study of the temperature dependence by coherent transients

The temperature dependence of the foreign gas relaxation of hydrogen cyanide, one of the best candidate for radioastronomical observations in planetary atmospheres, is investigated in the 135–300 K range. The measurement process exploits the delayed nutation phenomenon which leads to the determination of the population relaxation. This coherent transient technique avoids any deconvolution of the Doppler effect and a novel pulse sequence which alleviates the difficulties related to the finite saturation actually achieved is proposed. The great sensitivity of the method requires active gas pressures (≲10−4 Torr) much lower than in linewidth techniques, and thus results in an extension by about 60 K of the lower bound of attained temperatures. The millimeter Stark spectrometer used, driven by a microprocessor system, is temperature controlled by a cold gaseous nitrogen flow; its careful design allows a strong reduction of thermal transpiration effects, and thus leads to proper pressure measurements for worki...

Measuring the Boltzmann constant by mid-infrared laser spectroscopy of ammonia

We report on our ongoing effort to measure the Boltzmann constant,

Detailed profile analysis of millimetre 502 and 602 GHz N2O–N2(O2) lines at room temperature for collisional linewidth determination

k_B,

Absorption-line-shape recovery beyond the detection-bandwidth limit: Application to the precision spectroscopic measurement of the Boltzmann constant

using the Doppler broadening technique on ammonia. This paper presents some of the improvements made to the mid-infrared spectrometer including the use of a phase-stabilized quantum cascade laser, a lineshape analysis based on a refined physical model and an improved fitting program 2 increasing the confidence in our estimates of the relevant molecular parameters, and a first evaluation of the saturation parameter and its impact on the measurement of k B. A summary of the systematic effects contributing to the measurement is given and the optimal experimental conditions for mitigating those effects in order to reach a competitive measurement of

Experimental and theoretical studies of room-temperature sub-millimetre CH3 35Cl line shapes broadened by H2

k_B

Foreign gas and self-relaxation of CH3CN: Low-temperature dependence for the 92-GHz transition

at a part per million accuracy level are outlined.